Prior art reports of oxidative leaching using chlorine-based oxidising species typically describe the use of gaseous chlorine in acidic solutions, where the chlorine is either supplied from gas cylinders or generated in situ. However, chlorine is not an inexpensive reagent, and the consumption of aqueous chlorine is one of the most significant factors in determining the economic feasibility of chlorine-based oxidative leaching processes. The economic problem is exacerbated by the fact that methods for chlorine production generally exclude the use of a recycled solution, as high purity sodium chloride solutions are required for economical production, where recycled solutions will inevitably contain substantial concentrations of ions deleterious to chlorine production.
Further, with several economically significant ores/concentrates, chlorine-based leaching at low pH values has demonstrated limited utility. It has been postulated that this is due to the formation of a passivating layer on the ore.
Puvaada et al (Hydrometallurgy vol. 58, 2000, p. 185-191) leached a gold/silver bearing chalcopyrite concentrate in chloride/hypochlorite media. The starting solution typically contained 0.35 M (12.8 g/L) of HCl which is a pH of <1. The addition of 25-200 g/L NaCl improved the recovery. The final recoveries, were found to be very similar at all three NaOCl concentrations, with a maximum of 32.7%. The dissolution of silver at all levels of NaOCl increased equally steadily with time and attained a maximum recovery of 22.8%.
Consequently, they pre-oxidised the concentrate by heating it in solution to 150° C. under 10-25 atm of pressure. After this pre-oxidation, leaching of the aqueous pressure-oxidized copper concentrate with 25 ml/L NaOCl, 200 g/L NaCl and 0.35 M HCl resulted in enhanced gold and silver recoveries of 90.0% and 92.5%, respectively, in 1 h. Clearly, a pre-oxidation step was essential for high recoveries of gold and silver from the ore/concentrate during subsequent acid—chloride—hypochlorite leaching.
Slater (U.S. Pat. No. 1,438,869 A, 12 Dec. 1922) claims “A process of leaching metal values from material containing such values existing therein, in part at least, as sulfide, which comprises reacting upon such material with an acid leaching solution in excess, and thereafter reacting thereupon with a chlorin-oxygen compound in the presence of free acid.” The method described in Slater requires two separate leaches. First, Slater subjects ore to leaching in excess acid. The ore thus treated is then subjected to a “chlorin-oxygen” leach, also in acid solution.
CA 2478516 AL, 9 Feb. 2005 presents a process using hypochlorite as one of a long list of oxidants. This process is single stage, but utilises strong chloride solutions (>200 g/L MgCl2), high acidity (30-150 g/L HCl) and elevated temperatures (>75° C.). The process also aims to convert “sulfide sulfur that is leached from the sulfide ore material is converted into hydrogen sulfide”. The examples are all with solution pH of <0, the highest pH of operation within the claims is pH 2.5.
U.S. Pat. No. 2,205,565 A, 25 Jun. 1940 describes a process where sodium hypochlorite is used on “crude ore as mined, its concentrate or a partially roasted concentrate therefrom” to recover nickel and cobalt. The patent teaches that “it is important, when first applied to an ore that it have a high enough pH value, for example from 10 to 12, and be in sufficient quantity to provide the amount of oxygen required while alkalinity is maintained to oxidize all of the sulphides of nickel, cobalt and copper, together with a proportion of those of iron present in the ore, to sulphates before the solution can become sufficiently acid to prevent further oxidation due to destruction of hypochlorite with liberation of free chlorine.”
Cho (Leaching Studies of Chalcopyrite and Sphalerite with Hypochlorous Acid, Metallurgical Transactions B, Volume 18B, June 1987) describes laboratory studies of the effect of stirring speed, temperature, pH and hypochlorous acid concentration on the leaching of chalcopyrite and sphalerite over the pH range 3.6 to 5. The solution pH was controlled by the addition of sodium hydroxide. However, from a practical perspective, the addition of base generally, and sodium hydroxide in particular, will impact adversely on the economics of the leaching process, likely to the point of rendering such unviable. Additionally, the hypochlorous acid consumption is critical to economics and Cho reported 6.0-7.2 mol HClO per mol of copper in solution.
Further work by Cho et al (R. Garlapalli, E. H. Cho and R. Yang, Leaching of chalcopyrite with sodium hypochlorite, Hydrometallurgy 2008, eds. C. A. Young, P. R. Taylor, C. G. Anderson and Y. Choi, SME, p. 653-663) indicated that the leaching reaction had a maximum rate at pH 13 when using 0.5M hypochlorite at 85° C. The hypochlorite consumption was 20-65 mol HClO per mol of copper dissolved which is patently uneconomic.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
The discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.